PLASMA ADDRESSED LIQUID CRYSTAL DISPLAY The invention relates to a display screen comprising a substrate with parallel extending channels, filled with an ionizable gas, and provided on their walls with two parallel electrodes extending in the longitudinal direction of the channels for generating a gas discharge in the channels, said screen being further provided with a second substrate arranged parallel to the first transparent plate and provided with transparent picture electrodes directed transversely to the channels and enclosing, with the first substrate, a space which is filled with an electro-optical material.

During operation of such a display screen, the channels are rendered conducting one by one by generating a gas discharge in the channels. To this end, an ignition voltage, for example, a DC voltage of approximately 450 V is applied for a short time between the electrodes which are present on the wall of the relevant channel. After the gas discharge has started, the voltage between the electrodes on the wall of the channel is reduced to an operating voltage at which the gas discharge is maintained, in practice a DC voltage of, for example, approximately 250 V. This channel then forms a conducting electrode which is transparent because the electrodes in the channel only cover a part of the wall thereof. A voltage can now be applied between this conducting channel and one of the electrodes present on the second substrate (e. g. glass plak), so that the electro-optical material, for example, a liquid crystalline material which is present at the area of their crossing between the first and the second glass plate is influenced. Thus, a pixel of an image to be formed can be formed. In a corresponding manner, the liquid crystalline material can be influenced at every crossing of the channels and the electrodes on the second glass plate so that a pixel can be formed at each crossing. The channels define rows of pixels, the electrodes on the second substrate (e. g. a glass plate) define columns. Such a display screen, in which pixels are driven by a conductor formed by a gas discharge, is known as a PALC (Plasma-Addressed Liquid Crystal) screen. The electrodes on the second substrate are transparent and are made of, for example, indium tin oxide. The display screen may be further provided with polarization filters and possibly 32 color filters for visualizing the pixels.

As described hereinbefore, a DC voltage is applied, in operation, between the electrodes present on the wall of the channels. One of the electrodes forms a cathode while

the other forms an anode. The electrodes present on the wall of the channels are made of a satisfactorily conducting metal in practice. To ensure that the cathode is not attacked by the ions present in the gas discharge, it is coated with a protective coating in practice.

A display screen of the type described in the opening paragraph is known from EP-A-762 460, in which the electrodes are formed in a number of stacked layers of metal. A chromium layer is used as a sub-layer so as to ensure a satisfactory adhesion to the glass of the substrate. To obtain a satisfactory conductance, this chromium layer is provided with a layer of copper which, in turn, is coated with a top layer of chromium. To prevent attack of the electrodes by ions from the gas discharge, the electrodes are protected with a layer of a rare-earth hexaboride such as lanthanum hexaboride.

A similar display screen is known from EP-A-827 176, in which particles of an insulating material are added to the protective layer of the rare-earth hexaboride, or in which an extra layer of this insulating material is provided on this protective layer.

Magnesium oxide, silicon oxide or aluminum oxide is used as an insulating material. This additional or extra layer serves to increase the secondary emission of electrons of the cathode.

It has been found in practice that such a display screen has a relatively short lifetime, particularly when the ionizable gas is helium. In that case, it appears that the DC voltage with which the gas discharge can be started already becomes so high after approximately 1000 hours that the display screen may become unusable.

It is, inter alia, an object of the invention to provide a display screen of the type described in the opening paragraph with a considerably longer lifetime. According to the invention, the display screen is therefore characterized in that the wall of the channels next to the electrodes is provided with a barrier layer. This layer impedes transport of ions from the gas discharge to the glass of the substrate. Due to this measure, it appears that the lifetime of a display screen, in which the gas discharge is generated in helium, can be extended by a factor of 10. The barner layer may be transparent dependent on the kind of device.

The invention is based on the recognition that the large voltage which is applied between the electrodes on the wall of the channels so as to start the gas discharge results in an area with a negative space charge being generated around the cathode in the

substrate. Due to the high negative starting voltage on the cathode, atoms which are present in the substrate may be ionized; the positive ions then move towards the cathode where they leave said negative space charge area behind. The positive ions in the gas discharge are attracted by this area and disappear in the glass of the substrate. There, they appear to be bound to such an extent that they are no longer released during operation of the display screen. After approximately 1000 hours, so much helium has disappeared in the wall of a display screen, in which helium is used as an ionizable gas, that the gas discharge can no longer be started by said 450 V. The transparent barrier layer prevents ions from being drawn into the substrate. The lifetime of the display screen can thus be extended considerably.

The transparent barrier layer is preferably also provided between the electrodes and the wall of the channels. It is found that a part of the ions from the gas discharge impinging upon the negative cathode may also reach the substrate. This may also cause such a quantity of atoms of the gas to diffuse in to the substrate, so that the lifetime of the display screen is influenced detrimentally. By providing the transparent barrier layer also under the electrodes, the lifetime of said display screen can be additionally extended. During the formation of the electrodes, the wall of the channels is then protected by the barrier layer.

A satisfactory barrier layer comprises a layer of magnesium oxide, yttrium oxide or aluminum oxide. A layer of aluminum oxide having a thickness of between 100 and 250 nm extends the lifetime of said display screen from approximately 1000 hours to 15,000 hours.

It is found that, even when using the barrier layer, the lifetime can be still further extended when the electrodes are also coated with a non-conducting barrier layer which impedes transport of ions from the gas discharge to the electrodes. A very suitable non-conducting barrier layer is a layer of lead glass. The lifetime is thus additionally extended by another 20%.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings: Figs. 1 and 2 show diagrammatically a first embodiment of a display screen according to the invention in a plan view and in a cross-section, respectively, Fig. 3 is a diagrammatic cross-section of a second embodiment of a display screen according to the invention,

Figs. 4 and 5 are diagrammatic cross-sections on a larger scale of a channel of the display screen shown in Fig. 2, provided with barrier layers, and Figs. 6 and 7 are diagrammatic cross-sections on a larger scale of a channel of the display screen shown in Fig. 3, provided with barrier layers.

The Figures are diagrammatic and not drawn to scale, while the same reference numerals are used for corresponding construction parts.

Figs. 1 and 2 show diagrammatically a first embodiment of a display screen comprising a glass substrate 1 with parallel channels 2 closed at their upper side 3 by a first glass plate 4. In this embodiment, the substrate 1 is an approx. 2 mm thick plate of glass in which the channels 2 are formed by sawing. In this embodiment, the channels 2 have a width of approximately 400 um and a depth of approximately 200 pm, while their mutual distance is approximately 25 um.

The length of the channels 2 and the number of channels is dependent on the size of the display screen. In this embodiment, the first glass plate 4, also referred to as microsheet, is an approx. 50 um thick plate of glass of the type AF45 of the firm of Schott.

The substrate 1 is of the same type of glass so as to avoid mutual differences in expansion and accompanying mechanical stresses in the case of temperature differences.

The channels 2 are filled with an ionizable gas, in this embodiment, helium having a pressure of between 170 and 230 mbar.

On their wall 5, the channels 2 are provided with two parallel electrodes 6 and 7 extending in the longitudinal direction of the channels 2 for generating a gas discharge in the channels 2. In this embodiment, the electrodes consist of approx. 2 um thick and 50 llm wide aluminum strips coated with an approx. 3 um thick protective layer of lanthanum hexaboride. The electrodes may alternatively consist of nickel an assembly of layers having a sub-layer of chromium, an intermediate layer of copper and a top layer of chromium, which assembly is also coated with a protective layer of lanthanum hexaboride. One of the electrodes 6,7, for example, electrode 6, forms an anode, while the other electrode 7 forms the cathode. During operation, positive ions are attracted towards the cathode 7. The protective layer serves to prevent the cathode 7 from being attacked by ions from the gas discharge which is generated in the channels. In practice, such a protective layer may be provided on both electrodes 7 and 8.

The screen is further provided with a second glass plate 8 which is arranged parallel to the first glass plate 4 and is provided with transparent picture electrodes 9 directed transversely to the channels and enclosing, with the first glass plate, a space 10 which is filled with an electro-optical material. The electrodes on the second glass plate 9 are transparent and are made of, for example, indium tin oxide. The display screen is further provided with polarization filters 11 and 12 and possible color filters (not shown) for visualizing the pixels.

Fig. 3 shows a second embodiment of a display screen, in which the channels 2 are formed in a different way. Also in this case, the substrate 1 is a glass plate of approximately 2 mm. The electrodes 6 and 7 are formed on this plate. The electrodes 6 form the anodes and the electrodes 7 form the cathodes of the display screen. The anodes 6 have a width of approximately 200 pm, the cathodes 7 have a width of approximately 100 um. The anodes 6 and cathodes 7 have an equal thickness of approximately 2 Hm and are made of aluminum. The cathodes 7 are provided with a protective layer of lanthanum hexaboride.

Glass strips 13 are formed on the anodes by means of screen printing and by providing an paste of glass particles whereafter the glass is formed by means of a thermal treatment. The first glass plate 4 is then provided on the glass strips 13. The surface of the substrate 1 forms a part of the wall 5 of the channels 2. The channels 2 thus formed have a width of approximately 1000 pm and a height of approximately 500 m.

During operation of the two display screens shown, the channels 2 are rendered conducting one by one by generating a gas discharge in the channels. To this end, an initial voltage is applied for a short time between the electrodes 6 and 7 which are present on the wall 5 of the relevant channel. In this embodiment, the gas discharge is started by applying a DC voltage of 450 V for 2 psec between the electrodes 6 and 7. After starting the gas discharge, the voltage between the electrodes 6 and 7 is decreased to an operating voltage at which the gas discharge is maintained, in this example, a DC voltage of, for example, approx. 250 V. This channel then forms a conducting electrode which is substantially transparent because the electrodes 6 and 7 in the channel cover only a part of the wall 5 thereof. A voltage can now be applied between this conducting channel and one of the electrodes 9 present on the second glass plate 8, so that the electro-optical material, for example, a liquid crystalline material which is present at the area of their crossing in the space 10 between the first and the second glass plate is influenced. A pixel of an image to be formed can thus be formed. Fig. 1 shows very diagrammatically the display screen in a plan view, in which the channels 2 and the electrodes 9 extending transversely thereto are shown.

At each crossing of the channels 2 and the electrodes 9, the liquid crystalline material can be

influenced and a pixel can thus be formed at each crossing. In this embodiment, the channels 2 define rows of pixels while the electrodes 9 define columns of the image to be formed.

Figs. 4 and 6 show, for each of the display screens described above, one of the channels 2 in detail. These Figures show that the wall 5 of the channels 2, next to the electrodes 6 and 7, is provided with a transparent barrier layer 14 which impedes transport of ions from the gas discharge to the glass of the substrate 1. It is found that the lifetime of a display screen can be extended considerably by using such a barrier layer 14. As mentioned before electrodes 4,6 are coated with a coating 16 of LaB6.

The large voltage which is applied between the electrodes 6 and 7 for starting the gas discharge results in an area with a negative space charge being generated around the cathode 7 in the substrate 1. Due to the high initial voltage at the cathode, atoms present in the substrate may be ionized; the positive ions then move to the cathode where they leave said negative space charge area behind. This is particularly the case for glasses which can be used for the first glass plate 5, the approx. 50 um thin"microsheet". This glass should, inter alia, not have any interaction with the liquid crystalline material in the space 10. To prevent problems of thermal expansion, the substrate is made of the same glass. Such glasses contain barium which may easily lead to the afore-mentioned effects. The positive helium ions in the gas discharge are attracted by this area of negative space charge and associated atoms are absorbed in the glass of the substrate 1. There, they appear to be retained to such an extent that they are no longer released during operation of the display screen. After approximately 1000 hours, so much helium has disappeared in the wall in the display screen of this embodiment, in which helium is used as an ionizable gas, that the gas discharge can no longer be started by said 450 V. When the glass is heated to temperatures of approximately 500°C, it appears that it releases the built-in helium slowly again. This is of course no practical solution for extending the lifetime. The transparent barrier layer prevents ions from being drawn into the substrate. The lifetime of the display screen can thus be extended considerably.

The transparent barrier layer 14, as shown in Figs. 4 to 7, is also provided between the electrodes 6,7 and the wall 5 of the channels 2. By providing the transparent barrier layer 14 also under the electrodes 6,7, the lifetime of said display screen can be additionally extended. During etching of the electrodes 6,7, the wall 5 of the channels 2 is protected by the barrier layer 14. In this embodiment, the electrodes 6,7 are etched in a layer of aluminum in a customary basic etching bath.

Experiments on the two display screens described proved that the lifetime is approximately 1000 hours without a barrier layer. After 1000 hours, the gas discharge can no longer be ignited by applying a DC voltage of 450 V between the electrodes 6 and 7. With a barrier layer 14 provided as shown in Figs. 4 and 6, the lifetime is considerably longer. A 100 nm thick barrier layer of magnesium oxide, yttrium oxide or aluminum oxide extends the lifetime to approx. 5000 hours and a 250 nm thick layer of these materials extends the lifetime to more than 15,000 hours. A layer of aluminum oxide has the additional advantage that it is not attacked at all by the basic etching bath during etching of the electrodes 6 and 7 so that the underlying glass of the substrate 1 is well protected. Said barrier layers are vapor- deposited in the conventional manner, in which a closed and dense layer is formed.

It is found that, even when using the barrier layer, the lifetime can be still further extended when the electrodes, as shown in Figs. 6 and 7, are also coated with a non- conducting barrier layer 15 which impedes transport of ions from the gas discharge to the electrodes. A very suitable non-conducting barrier layer is a layer of lead glass. Said lifetimes are thus additionally extended by another 20%. This layer of lead glass is provided by means of electrophoresis. The coating 16 now overlies the complete electrodestructures and 6,15 and7,15.